The present invention relates to a strain meter which is used in the ground and in a solid construction. It is not so easy to make a measurement of a very -small expansion or contraction, because a signal caused by such displacement is too small to exceed the detection sensitivity of a detector, especially for a small size strain meter.
In order to measure strain in the ground and/or in a solid construction, the instrument and ground and/or solid construction must come into closer contact using cement. A cylindrical closed receptacle has generally been used for the strain meter in a borehole, making it impossible to install multiple instruments in the same borehole.